Field of the Invention
For many electronic systems, memories are needed into which the data can be written in fixed fashion, once and for all, in digital form. Such memories are known as OTP memories, among other names.
For large amounts of data, such as the digital storage of music or photographs, in particular, plastic disks, so-called compact disks, coated with aluminum are often used as memories. These disks have two kinds of dotlike indentations in the coating, which are assigned to the logical values of 0 and 1. The information is digitally stored in the arrangement of these indentations. The indentations are generated with the aid of a laser, for instance. This means that the compact disks are writeable once and for all.
To write or read a compact disk, the disk is mechanically rotated in a read/write unit. The dotlike indentations are scanned as the information is read via a laser diode and a photo cell. Typical scanning rates are 2.times.40 kHz. To write the information, the laser diode is operated at higher energy, causing a change in the absorbent layer of the compact disk and thereby forming an indentation. Approximately 5 Gbits of information can be stored on one plastic disk.
The read/write unit includes moving parts that wear mechanically, take up comparatively much volume, permit only slow data access, and consume large amounts of current. Moreover, the read/write unit is vulnerable to jarring and is therefore suitable for mobile systems only to a limited degree.
Electrically programmable semiconductor-based memories, especially of silicon, and known as EEPROMs or flash memories, are often used for storing smaller quantities of data. In electrically programmable memory cell arrays, the storage of the information is usually accomplished by providing, between the gate and the channel region of the MOS transistors, a floating gate that can be acted upon by an electrical charge or a double layer of SiO.sub.2 and Si.sub.3 N.sub.4 as a gate dielectric, at whose boundary layer electric charge carriers can be firmly captured at traps. The threshold voltage of the MOS transistor is dependent on the charge located at the floating gate or the adhesion points. This property is utilized for the electric programming (see for example S. M. Sze, Semiconductor Devices, John Wiley & Sons, pp. 486-490).
In readout of the memory cell array, the individual memory cells are selected via a word line. The gate electrode of each of the MOS transistors is connected to a respective word line. The input of each MOS transistor is connected to a reference line and the output is connected to a bit line. In the reading operation, it is assessed whether a current is flowing through the transistor or not. The logical values of 0 and 1 are assigned accordingly.
Technologically, the storage of 0 and 1 in these memories is accomplished in that the MOS transistors each have different threshold voltages, depending on the information stored in them.
These known silicon memories typically have a planar layout. This requires minimal surface area per memory cell, on the order of about 6 to 8 F.sup.2, where F is the smallest feasible Structural size for the particular technology employed. Planar read-only silicon memories are thus limited, in 0.4 .mu.m technology, to memory densities of around 1 bit/.mu.m.sup.2.